Have shoulder pain due to prosthetic surgeries. On MS Contin and MS IR. Having less pain. Cause?

I HAVE BEEN ON MS CONTIN FOR 7YRS. 60mgx6,2 TABS. 3X ADAY, ALSO 30 mg. MS IR 6 TABS PRN. I HAVE EXPERIENCED LESS AND LESS PAIN RELEIF OVER THE PAST 12 TO 18 MONTHS. I HAVE NOT HAD A CHANGE OR INCREASE FOR 4YRS. WHAT DO YOU THINK?

Hello,
Thanks for the query.
The chronic pain management drugs, have a tendency to become less effective over a period of time. This is called as tachyphylaxis. This condition is managed by either increase in dose or changing for drug for a small period of time and then coming back to the same drug. Please discuss with your treating physician regarding this. Let me know if you have any other doubts. Thank you.